Revealed: How Placebo Effect Works

Feb. 19, 2004 -- It's well known among doctors that people can get pain relief from a placebo. Now, they're closer to understanding the phenomenon called the placebo effect.

A new study provides evidence that a placebo -- the mere expectation of relief, with no real treatment -- causes physical changes in how the brain responds to pain, writes lead researcher Tor D. Wager, PhD, a psychologist with the University of Michigan in Ann Arbor. His paper appears in this week's issue of Science.

While the body still experiences the sensation of pain, the brain processes it differently when relief is expected -- the placebo effect, he explains.

In his study, Wager and his colleagues used functional MRI to view brain activity in about two dozen people.

Volunteers were told that they were testing a pain-relieving cream, which only the researcher knew was a placebo. The cream was applied, and the volunteers were given a shock while researchers viewed their brain activity.

The researchers wanted to see if the placebo "pain reliever" would alter the sensation of the pain in the brain. They could see this by viewing changes in pain-sensing regions of the brain.

When the placebo was used, the response of the brain's pain-sensing regions was ratcheted down.

These studies showed "placebo effect patterns" in the prefrontal cortex, writes Wager. The prefrontal cortex is the brain region that becomes activated in anticipation of pain relief -- which triggers a reduction of activity in pain-sensing areas of the brain.

This interplay within the prefrontal cortex may trigger a release of pain-relieving opioids in the midbrain, Wager speculates.

Another explanation of the placebo effect: The prefrontal cortex may direct attention away from pain, since the same region has been implicated in attention processes.